(*
Inductive relation "otway" for the Otway-Rees protocol
extended by Gets primitive.
Version that encrypts Nonce NB
*)
OtwayRees = Shared +
consts otway :: event list set
inductive "otway"
intrs
(*Initial trace is empty*)
Nil "[]: otway"
(** These rules allow agents to send messages to themselves **)
(*The spy MAY say anything he CAN say. We do not expect him to
invent new nonces here, but he can also use NS1. Common to
all similar protocols.*)
Fake "[| evsa: otway; X: synth (analz (knows Spy evsa)) |]
==> Says Spy B X # evsa : otway"
(*A message that has been sent can be received by the
intended recipient.*)
Reception "[| evsr: otway; Says A B X : set evsr |]
==> Gets B X # evsr : otway"
(*Alice initiates a protocol run*)
OR1 "[| evs1: otway; Nonce NA ~: used evs1 |]
==> Says A B {|Nonce NA, Agent A, Agent B,
Crypt (shrK A) {|Nonce NA, Agent A, Agent B|} |}
# evs1 : otway"
(*Bob's response to Alice's message. Note that NB is encrypted.*)
OR2 "[| evs2: otway; Nonce NB ~: used evs2;
Gets B {|Nonce NA, Agent A, Agent B, X|} : set evs2 |]
==> Says B Server
{|Nonce NA, Agent A, Agent B, X,
Crypt (shrK B)
{|Nonce NA, Nonce NB, Agent A, Agent B|}|}
# evs2 : otway"
(*The Server receives Bob's message and checks that the three NAs
match. Then he sends a new session key to Bob with a packet for
forwarding to Alice.*)
OR3 "[| evs3: otway; Key KAB ~: used evs3;
Gets Server
{|Nonce NA, Agent A, Agent B,
Crypt (shrK A) {|Nonce NA, Agent A, Agent B|},
Crypt (shrK B) {|Nonce NA, Nonce NB, Agent A, Agent B|}|}
: set evs3 |]
==> Says Server B
{|Nonce NA,
Crypt (shrK A) {|Nonce NA, Key KAB|},
Crypt (shrK B) {|Nonce NB, Key KAB|}|}
# evs3 : otway"
(*Bob receives the Server's (?) message and compares the Nonces with
those in the message he previously sent the Server.
Need B ~= Server because we allow messages to self.*)
OR4 "[| evs4: otway; B ~= Server;
Says B Server {|Nonce NA, Agent A, Agent B, X',
Crypt (shrK B)
{|Nonce NA, Nonce NB, Agent A, Agent B|}|}
: set evs4;
Gets B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
: set evs4 |]
==> Says B A {|Nonce NA, X|} # evs4 : otway"
(*This message models possible leaks of session keys. The nonces
identify the protocol run.*)
Oops "[| evso: otway;
Says Server B {|Nonce NA, X, Crypt (shrK B) {|Nonce NB, Key K|}|}
: set evso |]
==> Notes Spy {|Nonce NA, Nonce NB, Key K|} # evso : otway"
end